Synthesis and characterization of amorphous Al₂O₃ and γ-Al₂O₃ by spray pyrolysis

1 Introduction

Alumina has abundant applications in the ceramics industry and heterogeneous catalysis, and it has been widely used as absorbents, abrasive materials, ceramics, and biomaterials [1–5]. To achieve such capabilities, alumina should have unique properties, such as high refractive index [6], high dielectric constant, chemical and thermal stability [7], and high transparency [4].

Different and contradictory properties of alumina make it versatile according to our tendency. For example, we can use it for optoelectronic applications; good homogeneity with both good density and dielectric characteristics as well as a low surface roughness [8] make alumina an important material for the ceramic industry. It has been found that amorphous Al₂O₃ has extensive application in catalysis, coatings, microelectronics, and thin film devices because of their chemical, thermal, and mechanical stability [9–11]. At the same time, γ-Al₂O₃ is an important mesoporous material, which has been intensively studied with regard to technical applications as catalysts and catalyst supports [12, 13]. It has been extensively used as advanced catalysts and catalyst supports because of its low cost, good thermal stability, and high specific surface area [14, 15].

Commercial alumina is usually manufactured by the precipitation of aluminum salts [16], sol-gel processing of aluminum alkoxides [17, 18], powder processing technology using various inorganic, and organic aluminum precursors [19–21]. Compared with the previously mentioned method, spray pyrolysis is the result of the decomposition of single inorganic precursors in the air, which is a “green” method. At the same time, hydrogen chloride produced by the exhaust gas is absorbed directly, so the entire process has the advantage of short residence time, high production efficiency, low operating cost, and energy consumption. During the preparation process, atomized droplets of a precursor solution undergo evaporation and shrinkage while flowing through a high-temperature reactor and eventually form particles. Because of evaporation, precipitation, drying, and decomposition occurring in a dispersed phase and a single step, it becomes possible to control the key particle properties (size, morphology, chemical composition, etc.) easily by controlling the process parameters (residence time and decomposition temperature) [22–27]. The spray pyrolysis technique is based on the ultrasonic generation of micrometric-sized aerosol droplets and their decomposition at intermediate temperatures (273–1073 K). Therefore, this method is a continuous flow process and is more economical and more green than other approaches (such as sol-gel processes) that involve multiple steps [28–31].

From an industrial point of view, rare earth elements are important owing to their unique properties and wide range of applications in various fields such as electronics, magnetism, metallurgy, phosphors, catalysts, glass, laser, and ceramic technology [32–34]. Because La³⁺ has more structural defects, the addition of La can enhance the thermal stability of γ-Al₂O₃ [35], and it was added in the amorphous Al₂O₃ to study the influence of La on the thermal stability of it.

The present work focuses on the mass production of high surface area Al₂O₃ nanoparticles using inexpensive precursors developed from natural minerals, such as bauxite, and the effect of the precursor on the pore structure of alumina prepared by the spray pyrolysis method [19, 36]. In this work, the amorphous and the gamma crystal types of alumina were obtained by controlling the respective conditions. Microstructural and morphological analyses were conducted by using X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy, and infrared (IR) absorption spectrum.

2 Materials and methods

2.1 Amorphous Al₂O₃ particle synthesis

Solutions of AlCl₃·6H₂O (AR >99.0%; Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) were used as the precursors. The concentration of the precursors was 20.0 wt%. The schematic representation of the spray pyrolysis equipment (Northeastern University in Shenyang, Liaoning Province, China) shown in Figure 1 is as follows: the spray pyrolysis system consisted of a homemade atomizer, a corundum tube located inside a tubular furnace, three cyclones as the collectors, a gas buffer tank, and a tail gas absorber. Precursor droplets were sprayed by expanding compressed air through the atomizer. After passing through the diffusion dryer, these partly dried droplets were carried into the corundum tube housed inside the tubular furnace, and the resultant pyrolysate was finally collected by a filter sampler. The temperature of the tubular furnace was controlled and adjusted in the range of 1073–1273 K. Particle residence time in the roaster was controlled by varying the airflow rate from 2.0 to 3.2 l min^-1, corresponding to a residence time of 0.7–1.0 s and a collector temperature of 273–278 K by an ice water bath.

Figure 1:

Schematic diagram of automated spray pyrolysis experimental device. (1) Corundum tube, (2) tubular furnace, (3) the first-stage cyclone, (4) the first-stage collector, (5) the second-level cyclone separator, (6) the second-stage collector, (7) the third cyclone, (8) the third-stage collector, (9) flow controllers, (10) tail gas absorber, (11) lifting device, (12) air compressor, (13) into the tank, (14) solution, (15) hose, (16) injection pipe, (17–19) water bath temperature control.

3 Results and discussion

Figure 2 shows the XRD patterns of samples obtained by pyrolysis at different temperatures. It can be seen that the XRD patterns consist of three broad diffuse peaks between diffraction angles 20° and 80°. The sample obtained at a temperature of 1073 K (Figure 2A) shows some small shape peak, which indicates some crystalline phase mixed with the primary amorphous structures. When the pyrolysis temperature is up to 1173 and 1273 K, there is no apparent crystalline phase corresponding to the sharp crystallization peak in (Figure 2B and C), indicating that this sample is in the amorphous structures. As the pyrolysis temperature is 1173 K, its pattern shows broad diffuse backgrounds, but the amorphous diffuse peak is shape, and the amorphous diffuse peak of 1173 K is sharper than that of 1273 K, showing that it has the trend of further crystallization. Thus, the threshold overheating temperature for the fully amorphous structure of amorphous Al₂O₃ is at least 1173 K, below which it may have an intersection with the crystallization position.

Figure 2:

XRD patterns of amorphous Al₂O₃ at different pyrolysis temperatures: (A) 1073 K, (B) 1173 K, (C) 1273 K, collector temperature at 273–283 K, and particle residence time in the calciner for 0.7–1.0 s.

Figure 3 depicts XRD patterns of the Al₂O₃ particles modified by La³⁺, and no crystalline peak is detected from all particles obtained with each La³⁺ concentration. Thus, the particles can be considered as amorphous structure because La³⁺ has more structural defects, which can effectively improve the stability of amorphous Al₂O₃.

Figure 4 presents the XRD patterns of γ-Al₂O₃ at different pyrolysis temperatures. As the pyrolysis temperatures are 1373 and 1273 K, their pattern peaks are identified as γ-Al₂O₃ peaks at 19.44°, 37.59°, 39.47°, 45.84°, and 67.00°. When the pyrolysis temperature is down to 1173 K, there is no apparent crystalline phase corresponding to the sharp γ-Al₂O₃ peak, indicating that this sample have a certain percentages of the amorphous structure.

Figure 3:

XRD patterns of amorphous Al₂O₃ modified by different La³⁺ concentrations (A) LaOCl 1 wt%, (B) LaOCl 2 wt%, (C) LaOCl 3 wt%, pyrolysis temperatures of 1073 K, collector temperature of 273–283 K, particle residence time of 0.7–1.0 s.

Figure 4:

XRD patterns of γ-Al₂O₃ at different pyrolysis temperatures: (A) 1173 K, (B) 1273 K, (C) 1373 K, collector temperature of 293–318 K, and particle residence time of 0.7–1.0 s.

Figure 5 shows that the TEM images of the amorphous Al₂O₃ (Figure 5A and B) were obtained at a pyrolysis temperature of 1273 K, a residence time of 0.7–1.0 s, a collector temperature of 273–278 K by ice water bath and γ-Al₂O₃ (Figure 5C and D) at pyrolysis temperature 1373 K, a residence time of 2.6–3.0 s, and a collector temperature of 293–318 K by water bath. It can be seen from the TEM images that the Al₂O₃ particles exhibit a flake particle with a particle size distribution between 5 and 20 μm. The main reason is that droplets would turn into spherical shells at a high temperature, and these shells may burst into fragments because of the gas generated in them. In Figure 5B, the diffraction pattern is an amorphous ring proving that alumina crystals did not grow, which is consistent with XRD diffraction diagram. Figure 5D shows a selected area electron diffraction pattern taken from the sample with a pyrolysis temperature of 1373 K and a residence time of 2.6–3.0 s, indexed in accordance with the γ-Al₂O₃ phase (JCPDS=29-0063), having a defect spinel structure [37, 38].

Figure 5:

TEM images and SAED patterns of the amorphous Al₂O₃ at 1273 K (A and B) and γ-Al₂O₃ samples at 1373 K (C and D).

The FT-IR spectra of amorphous Al₂O₃ prepared by spray pyrolysis are shown in Figure 6. The band at 3457 cm^-1 is due to the stretching mode of hydroxyl groups (from surface water and adsorbed water). The band at 1643 cm^-1 is due to the bending mode of water molecules, and the content forms La³⁺ during the Al₂O₃. Then the hydrolysis of the La³⁺ cation takes place, and the resulting protons generate Brønsted acid sites. Therefore, the amount of Brønsted acid sites is increased. The broad band at 755 cm^-1 can be ascribed to the Al-O vibration of (AlO₄) [39, 40]. As La is added in the aluminum chloride solution, pyrolysis occurs at 1073 K instead of crystallization. Therefore, adding La can effectively inhibit crystallization because of slow cooling. The experimental results are consistent with XRD analysis.

Figure 6:

FT-IR spectra of amorphous Al₂O₃ (A) pyrolysis temperature at 1273 K, residence time of 0.7–1.0 s, collector temperature of 273–278 K; (B) 1173, 0.7–1.0 s, 273–283 K; (C) 1073, 0.7–1.0 s, 273–283 K, LaOCl 3 wt%; (D) 1073, 0.7–1.0 s, 273–283 K, LaOCl 2 wt%; (E) 1073, 0.7–1.0 s, 273–283 K, LaOCl 1 wt%.

The SEM images and the EDS patterns of the amorphous Al₂O₃ and γ-Al₂O₃ are shown in Figure 7. We can see that the Al₂O₃ sample is a flake particle with a particle size distribution between 5 and 20 μm, which is consistent with the TEM images. The EDS examination confirms the presence of Al and O from the sample, and the existence of C elements is due to the conductive adhesive.

Figure 7:

SEM images and EDS patterns of the amorphous Al₂O₃ at 1273 K (A and B) and γ-Al₂O₃ samples at 1373 K (C and D).

4 Conclusions

By changing the pyrolysis temperature, the particle residence time in the roaster, and the temperature of the collector, the amorphous Al₂O₃ and the γ crystal structure of Al₂O₃ were successfully prepared by spray pyrolysis. The effect of the pyrolysis temperature, the particle residence time in the roaster, and the temperature of the collector on the crystal shape is very obvious. Previous studies showed that La³⁺ can effectively improve the stability of the γ-Al₂O₃, and it was found to be helpful to the stability of the amorphous Al₂O₃ in this work. The proposed spray pyrolysis to synthetic Al₂O₃ was simple, cheap, efficient, and free of pollution, which makes it very suitable for scale-up production. Furthermore, LaAlO₃ can be made with amorphous Al₂O₃ and microcrystalline LaOCl solid solution precursor by this “green” synthetic method, which will be mentioned in another article.